import 'package:flutter/material.dart';
import 'package:flutter/widget_previews.dart';

@Preview(name: '我的示例文本')
Widget mySampleText() {
  return const Text('你好，世界！');
}

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // TRY THIS: Try running your application with "flutter run". You'll see
        // the application has a purple toolbar. Then, without quitting the app,
        // try changing the seedColor in the colorScheme below to Colors.green
        // and then invoke "hot reload" (save your changes or press the "hot
        // reload" button in a Flutter-supported IDE, or press "r" if you used
        // the command line to start the app).
        //
        // Notice that the counter didn't reset back to zero; the application
        // state is not lost during the reload. To reset the state, use hot
        // restart instead.
        //
        // This works for code too, not just values: Most code changes can be
        // tested with just a hot reload.
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        // TRY THIS: Try changing the color here to a specific color (to
        // Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
        // change color while the other colors stay the same.
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Container(
          width: 200,
          height: 200,
          child: Form(
            key: _formKey,
            child: Column(
              children: [
                DropdownMenuFormField<String>(
                  dropdownMenuEntries: const [
                    // 定义下拉选项
                    DropdownMenuEntry(value: 'red', label: 'Red'),
                    DropdownMenuEntry(value: 'green', label: 'Green'),
                    DropdownMenuEntry(value: 'blue', label: 'Blue'),
                  ],
                  validator: (value) {
                    // 内置表单验证！
                    if (value == null) {
                      return 'Please select a color';
                    }
                    return null;
                  },
                ),

                SizedBox(height: 33),
                ElevatedButton(
                  onPressed: () {
                    if (_formKey.currentState!.validate()) {
                      // 处理表单提交

                      print('Selected color: $_selectedColor');
                    }
                  },

                  child: const Text('Submit'),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }

  // 1. 定义一个 Form 和 GlobalKey

  final _formKey = GlobalKey<FormState>();

  String? _selectedColor;
}
